7 In humans the circulation behaves in many ways as if it consisted of two parts. In the systemic circulation the left ventricle pumps oxygenated blood through the organs and tissues of the body, where oxygen is removed and carbon dioxide added before the blood returns to the right atrium. In the pulmonary circulation the right ventricle pumps deoxygenated blood through the lungs, where oxygen is added and carbon dioxide is removed (Fig. 7.1). Of course, the two parts of the circulation work in series to form a single circuit of blood, but there are a number of differences between the pulmonary and systemic circulations which reflect their different functions. Every minute, the same volume of blood – about 5 L at rest – flows through both the right ventricle and the left ventricle. However, the two ventricles look very different (Fig. 7.2). The left ventricle has a thick, muscular wall that takes up most of the cross-section of the heart. The right ventricle has a much thinner wall, about one-third of the thickness of the left, and to accommodate the muscle of the left ventricle the right almost seems to be ‘wrapped around’ the left. Why is there such a difference between the two? The reason is that the left ventricle pumps blood into the systemic circulation, which has a high resistance and which operates at a relatively high pressure. If cardiac output increases, for example during exercise, this pressure can increase even more, which means that the left ventricle needs a thick, muscular wall to produce these high pressures. On the other hand, the right ventricle pumps blood into the pulmonary circulation, which has a very low resistance and which operates at a low pressure, always less than that in the systemic circulation. Furthermore, as we shall see later, if cardiac output increases the pulmonary artery pressure does not increase very much. For this reason, the right ventricle needs only a relatively thin muscular wall in comparison to the left. As well as having thinner walls, the pulmonary vessels are much more distensible than systemic arteries, which is important in keeping pulmonary blood pressure low during systole and in the face of increases in cardiac output. In circumstances such as exercise, cardiac output can increase from its normal 5 L per minute to as much as 25 L per minute. In order to keep pulmonary blood pressure low, the pulmonary circulation is able to reduce its resistance to even lower values than normal. It does so by two mechanisms, illustrated in Figure 7.3. 1. Pulmonary blood vessels are able to dilate or distend. A small increase in the diameter of a vessel decreases the resistance of that vessel substantially (Poiseuille’s Law, p. 156). 2. During normal conditions, some blood vessels in the lungs are closed. During periods of high cardiac output these vessels open up and blood is able to flow through them. The increase in number of blood vessels able to carry blood is called recruitment. However, it is likely that very few vessels are completely collapsed under normal conditions, and distension is probably much more important than recruitment in reducing vascular resistance. The anatomical position of the capillaries and their function in gas exchange means that they are different from their systemic counterparts. The density of the capillary network in the alveolar walls is extremely high so that efficient gas exchange can take place. In fact, there are so few cells between the capillaries in the alveolar walls that the alveolar circulation behaves almost like a film of blood flowing around the alveoli. At rest, blood flows through an alveolar capillary in about 0.8 seconds, which is about three times longer than the time needed for oxygenation of mixed venous blood. There is very little space between the blood in the pulmonary capillaries and the air in the alveoli. In fact, for the most part, the only cells separating blood from air are the endothelial cells of the pulmonary capillary and the epithelial type I cells of the alveolar wall (Fig. 2.9). This is clearly very important in allowing the most efficient transfer of gases between the alveoli and the blood (see Chapter 6). Not all the blood flowing into the lungs does so via the pulmonary artery – a small volume of arterial blood flows through the bronchial circulation (Fig. 7.4). The bronchial circulation supplies blood to the airways and to the lung parenchyma, although it is not essential to their survival; during a lung transplant the bronchial circulation is not reconnected, and this apparently does not produce any serious ill effects. Blood flowing through the bronchial circulation does not pass through the alveolar capillaries and therefore does not take part in gas exchange. However, the blood from the more distal parts of the bronchial circulation drains via the deep bronchial veins into the pulmonary circulation. This blood therefore forms a shunt (see below). In other words, the blood arises from the aorta, but instead of draining into the right-hand side of the circulation, the partly deoxygenated blood of the deep bronchial veins drains into the oxygenated blood that has passed through the alveolar capillaries. The resulting mixture of blood therefore has an oxygen content less than that of the original pulmonary artery blood. The significance of this will be discussed later in the chapter. The diastolic blood pressure in the systemic circulation is about 80 mmHg, which is enough pressure to raise a column of water by a height of over a metre. In other words, there is more than enough pressure to carry blood from the heart up to the head. However, in the pulmonary circulation the diastolic blood pressure is about 12 mmHg, enough pressure to raise a column of water about 15 cm. In other words, there is only just enough pressure to pump blood from the right ventricle up to the lung apices. On the other hand, at the lung bases the blood pressure in the pulmonary circulation is equal to the pressure generated by the right ventricle plus the hydrostatic pressure of a column of blood extending up to the heart. Because the pressure generated by the right ventricle is not very high, this extra hydrostatic pressure makes a very significant difference. Thus there is a very considerable difference in arterial blood pressure between the bases and the apices of the lungs owing to gravity. In other words, gravity tends to direct blood towards the lung bases. Case 7.1 The pulmonary circulation: bringing blood and gas together: 2 What causes a pulmonary embolus and how can it be diagnosed? A pulmonary embolus occurs when something, usually a thrombus (blood clot), occludes part of the pulmonary artery tree. Generally the thrombus forms in the veins of the pelvis or lower limb, and part of that thrombus or the whole thrombus may dislodge and pass through the vena cava, through the right atrium and ventricle and into the pulmonary artery. The thrombus finally lodges in a branch of the pulmonary artery, occluding it. The segment of lung tissue supplied by the obstructed artery has a reduced blood supply (although it often receives some blood – remember the bronchial circulation) and may finally infarct. In a small number of cases (probably less than 10% of the total number of pulmonary emboli) the thrombus does not form in the veins of the pelvis or leg but forms in the heart. This may be as a result of atrial fibrillation, in which the atria of the heart do not beat properly, or thrombus may form on a part of the myocardium which has infarcted. Very occasionally the embolus is not formed from thrombus but from other substances, such as fat or amniotic fluid. Small emboli in the lungs cause no symptoms and no haemodynamic problems and go unnoticed. Larger emboli, particularly if they result in pulmonary infarction, can cause clinical symptoms, including pleuritic chest pain and sometimes haemoptysis. Very large emboli are a medical emergency (see Box 4).
THE PULMONARY CIRCULATION
BRINGING BLOOD AND GAS TOGETHER
The anatomy of the pulmonary circulation
The right ventricle
Pulmonary blood vessels
The bronchial circulation
Distribution of blood flow through the lungs
Gravity
Stay updated, free articles. Join our Telegram channel
Full access? Get Clinical Tree
ratio, varies by a relatively small amount throughout the lungs.
ratio in the lungs affects arterial blood gases.
ratio as possible.